1. Field of the Invention
The present invention relates to a vacuum carburizing method of parts or workpieces made from steel material.
2. Description of the Related Art
Conventional carburizing method of parts or workpieces made from steel material are widely employed for example as gas carburizing, vacuum carburizing, or plasma carburizing methods. However, gas carburizing has the problems such as safety problems due to the combustibility of the gas, a generation of grain boundary oxidation of the carburizing surfaces of parts or workpieces treated and the problem to shorten the process cycle time for the difficulty in the carburizing treatment at a high-temperature. On the other hand, plasma carburizing requires high equipment costs and is restricted to special carburizing treatments such as those of parts or workpieces having deep holes.
The conventional vacuum carburizing is performed by heating workpieces made from steel material to about 900-1000.degree. C., and introducing a gaseous saturated aliphatic hydrocarbon including methane gas, propane gas and butane gas as a carburizing gas at a pressure of 10-70 kPa in a carburizing furnace chamber. However, the latter vacuum carburizing suffers from disadvantages including its high equipment costs, difficulty in the maintenance of the furnace due to the production of a large amount of soot and its high operating costs.
In order to reduce the operating costs of the conventional vacuum carburizing treatment, U.S. Pat. No. 5,702,540 proposes a vacuum carburizing treatment in which carburizing treatment is performed by vacuum heating workpieces from steel material in the heating chamber of a vacuum carburizing furnace, and supplying a carburizing gas to the heating chamber, employing a gaseous unsaturated aliphatic hydrocarbon comprising an acetylenic gas as the carburizing gas, and performing the carburizing treatment with the heating chamber at a vacuum of not more than 1 kPa. However, the acetylenic gas is explosive when pressure is applied, and it is difficult and troublesome to handle. Further, since a large amount of soot is produced when operated under a high vacuum pressure, the pressure is set at the vacuum of not more than 1 kPa, resulting in the need of a large volume vacuum pump or a mechanical booster pump, which leads to complicated installations and excessive equipment costs.
Further, U.S. Pat. No. 5,205,873 discloses a vacuum carburizing method for metal alloy parts using a fuel mixture gas consisting of hydrogen and ethylene gases as a carburizing gas at a pressure of 1-10 kPa in a carburizing furnace chamber. However, since this method uses lightly explosive hydrogen gas, it also suffers from disadvantages including its high equipment costs, a long process cycle and difficulty in handling so that it requires an introduction of nitrogen gas into the heating furnace chamber prior to the loading and heating the metal alloy parts.